The present invention relates to power factor correction (PFC) methods and circuits. PFC circuits are known in the literature and are applied in a broad range of electronic equipment, ranging from lighting ballast circuits, to switch mode power supplies, to motor drives.
Such types of equipment generally require a direct current (DC) input voltage to operate properly, and the DC voltage is usually obtained by rectifying and filtering an alternating current (AC) line voltage.
However, any time a non linear load (like an input rectifier bridge followed by a filtering capacitor) is applied to the AC line, such load produces a distortion of the line current from being sinusoidal and in phase with the line voltage.
Such distortion has to be limited for several reasons, but mainly to reduce the losses in the line distribution network as well as the electromagnetic interference (EMI).
Various countries (especially Europe) have issued regulations which define limits to the harmonic content of the current absorbed by any kind of equipment connected to the main distribution network (the AC line), with classifications which apply to different equipment and diversely limit such harmonic content.
For that reason, various types of power factor or harmonic correction circuits (active or passive) have been developed which, added at the input of the equipment, make the equipment compliant with such regulation limits.
For non cost sensitive applications, a properly controlled boost PFC topology is usually applied. Such topology is able to shape the input line current to be substantially sinusoidal and in phase with the line voltage, and is able to regulate quite well the output DC voltage versus load (the power drawn by the equipment) and line variations, but has the following drawbacks:                a) complex and expensive power electronic circuitry is required;        b) high frequency operation generates high frequency EMI;        c) the circuitry acts as a “series” connected active filter in between the rectified line voltage and the equipment to be fed, thereby transmitting all the power drawn by the equipment; and        d) due to that, and due to the high frequency operation, the boost PFC introduces power losses which, often, are not negligible with respect to the total power losses of the equipment to be fed. For a motor drive, for example, electrical to mechanical energy conversion efficiency is greatly reduced.        
On the other hand, cost sensitive applications which handle limited amounts of power (below 1 HP) try to avoid the use of such expensive active PFC methods and only use passive filters (usually inductors) in series with the line (before or after the rectifying bridge) at the input of the equipment.
Such “passive” filters, have several drawbacks:                a) they are bulky (and sometime expensive);        b) they also transmit all the power consumed by the equipment, so dissipate significant power;        c) they constitute a series impedance at the equipment's input, thereby reducing the DC voltage available at the equipment itself; furthermore, the voltage drop generated by such series impedance changes with the load. For example: for a motor drive, if the motor requires more torque (hence current) at high speed, the voltage available to the motor goes down (while, at high speed, more voltage would be required).        
In the recent past, some “low switching frequency” or “line frequency” active filters have been proposed.
One example is reported in the paper: “A Double-Line-Frequency Commutated Rectifier Complying with IEC-1000-3-2 Standards” by Jose' Antenor Pomilio, School of Electrical and Computer Engineering, University of Campinas (Brasil) and Giorgio Spiazzi, Department of Electronics and Informatics, University of Padova (Italy). Such methods have the general advantage that only part of the power drawn by the equipment is transmitted by the “active” filter, which then dissipates very little power itself. Also, the main filter inductor may be much smaller and cheaper when compared to the one used in a simple passive filter. The circuit switches at line frequency, so power dissipation due to switching losses is further greatly reduced. Finally, such methods generally provide quite good regulation of the output voltage of the filter stage versus load, but, when the auxiliary switch is being driven only once for every line half period as described in the paper, have the general drawback that regulation of the output voltage versus the input (line) voltage variations is very poor.
Another aspect to be considered in cost-sensitive applications is the housekeeping power supply for peripheral circuits such as logic and control circuits. Generally speaking, both the harmonic correction circuit logic or control circuitry and the logic or control circuitry of any equipment/power stage which is fed by the harmonic correction circuit needs a low voltage DC power supply. Such low voltage supply is usually derived by a dedicated circuit (very often a flyback switching power supply directly fed by the rectified line but sometimes a step-down switching regulator) which adds a significant cost to the whole power conversion chain, and, by switching at high frequency, introduces other EMI. In other cases, a simple resistive drop is used, which dissipates much more power than the power it actually delivers to the housekeeping circuitry.